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B Do we understand the cosmos

  1. Nov 14, 2016 #1


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    According to this paper there are many unanswered questions in cosmology.
    Do We Really Understand the Cosmos?
    T. Padmanabhan
    Comments: Invited Review; 28 pages; 1 figure
    Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)
    Our knowledge about the universe has increased tremendously in the last three decades or so --- thanks to the progress in observations --- but our understanding has improved very little. There are several fundamental questions about our universe for which we have no answers within the current, operationally very successful, approach to cosmology. Worse still, we do not even know how to address some of these issues within the conventional approach to cosmology. This fact suggests that we are missing some important theoretical ingredients in the overall description of the cosmos. I will argue that these issues --- some of which are not fully appreciated or emphasized in the literature --- demand a paradigm shift: We should not think of the universe as described by a specific solution to the gravitational field equations; instead, it should be treated as a special physical system governed by a different mathematical description, rooted in the quantum description of spacetime. I will outline how this can possibly be done.
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  3. Nov 14, 2016 #2
    OK, but is this not simply a statement of what already has been concluded?
    That GR is not compatible with QM, although GR does provide a good description of how gravity works on the large scale.
    So therefore we need a quantum theory of gravity to reconcile this.
    There already are proposals for this, a popular one is loop quantum gravity.
    However none of the proposals so far (that I know of), have yet led to a prediction which could be tested.
  4. Nov 14, 2016 #3


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    I really don't get some of these critiques.

    For example:
    "Expansion is in the eye of the beholder"

    Yes, it definitely is. The uniform expansion we observe is dependent upon selecting a specific class of observers (who view the universe as approximately homoegenous and isotropic). Obviously if you pick different observers, you'll get a universe that looks different. The fact remains that the uniform expansion isn't a feature found in most space-times. He tries to claim the last statement is false by dropping the "uniform" part of the expansion, showing that there's a sort of expansion going on for geodesic observers in a Schwarzschild space-time. This "expansion", however, is decidedly not uniform.

    "Everything is allowed in cosmology"

    Here he seems to be complaining that there are no limits imposed on the relationship between pressure and density. Well, of course not: the relationship between pressure and density depends upon the contents of the universe. There may be a fundamental limit that requires ##p(t)/\rho(t) \ge -1##, and I'm not aware of a type of matter which has ##p(t)/\rho(t) > 1/3##, but that's about it. I don't see why this is at all an issue. It just says that we need to measure the contents of the universe (i.e., what makes up the stress-energy tensor) in order to fully-specify the space-time.

    "Come back aether, all is forgiven?"

    Uh, what? It seems clear to me that the likely explanation for this feature is some spontaneous symmetry breaking event in the very early universe. That means that this particular problem just reduces to the spatial curvature and horizon problems of the classical big bang theory (which are only partially solved by inflation).

    Anyway, I think I'm done for now. I don't understand why most of these should be considered significant issues, and many of the others reduce to other issues that cosmologists talk about all the time. It seems clear to me that Padmanabhan really would like there to be some way to think very clearly and from that clear thinking deduce all of the observed features of our universe. I strongly doubt that will ever be possible.
  5. Nov 17, 2016 #4


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    This sounds so much like the prologue to a crank idea, I have the vague feeling of having just watched an episode of unsolved mysteries.
  6. Nov 17, 2016 #5


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    Padmanabhan is a professional theoretical physicist, and has a good amount of work under his belt, and is generally pretty highly-regarded. I think he's wrong here, but I don't think I'd go so far as to suggest it's crankery.

    Theorists often develop very strong beliefs about the importance of the very fine details of how we think about the universe, and that's what I see mostly at work in this paper. I think that developing such a perspective can be extremely useful for the advancement of theoretical physics, but it's extraordinarily difficult to do it without making a number of unfounded assumptions.
  7. Nov 30, 2016 #6
    The paper "Do we understand the Cosmos" raises a very pertinent question and provides a partial set of answers and alternatives. It is not a radically different approach to cosmology. It merely indicates that many of the beliefs that one holds regarding the universe, in terms of Einsteinian models, inflationary models etc, have to be questioned as a part of our model building. Sections (2),(3),(4),(5) and (6) indicate the open questions underlying the current cosmological models, the validity of tacit assumptions that are oft taken for granted and the overall conceptual basis for the holistic model for the universe based on GR, Inflation and expansion. The general feeling that the universe is as observed because it is expanding and everything is relative with no absolute frame of rest, and is controlled by a mysterious cosomological constant is a deep seated 'conditioning' that is worthwhile to question.

    b) Sections (7) and (8) provide an answer to the questions raised in the earlier sections 2-6 via a simple alternative model, a reinterpretation of GR and coupling it to an information based description of the universe. The answers provided lead to a distinctly falsifiable/testable model of the universe which addresses all of the observational properties of the present universe.

    The criticism of a scientific paper must be based on rigorous scientific principles and there is no place for the notion of 'crankery' etc. It is quite possible for even a 'crank' to make a highly correct and pertinent observation or question the universe ( the tale of Emperor's new clothes comes to mind). The casual dismissal of a carefully constructed scientific argument which has been in vogue for much of the history of human civilization is antithetical to the process of science. Irrespective of who writes the paper, and what the prevailing ideas of science is, the only validation of a scientific paper should be in terms of its assumptions, its methodology, consistency of thought and technique, correspondence with observations etc. Whether it challenges or contradicts one's pet theory or the universally held views is not germane to the quality of a scientific paper. If so it would be more akin to a faith based system such as 'religion'.

    The important thing is that the approach solves the cosmological constant problem which nobody else has done. Eqns 35 and 37 are the key results on the basis of which the paper should be judged. The rest of the paper provides a context/paradigm in which this result can be embedded.
  8. Nov 30, 2016 #7


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    That's an ideal situation, but I don't think it matches reality. Sadly, it sometimes happens that a wildly unscientific or ignorant paper gets published. I don't think that this is the case here: I think that Padmanabhan is arguing for a form of neo-Platonism to guide theoretical thinking. While that may be useful as a motivator for new theoretical investigations, I don't think it's a good measure of the truth. The problem is that arguments like the one that Padmanabhan laid out always come with assumptions, and those assumptions are usually implied rather than clearly stated (not because Padmanabhan did anything wrong, but because it's really, really hard to identify all of the assumptions explicitly).

    So I'd just say that it's probably useful for some people to examine the universe using the perspective laid out by Padmanabhan, but it's equally useful to have others who make use of other theoretical perspectives. Eventually we hope that observations will resolve any discrepancies (though sadly this may not always be the case).
  9. Nov 30, 2016 #8


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    Indeed. And this paper must be held to those same high standards, of which Chalnoth and others are doing.
  10. Dec 1, 2016 #9


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    Somehow, with these Padmananhan-papers I often wonder what I have learned after all those pages.

    Understanding is a relative concept. Compared to what? Compared to a century ago, I'd say "yes". If we have solved the questions mentioned in the paper, new ones will probably lure around the corner.
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